Can i wire a different volume control to the sub of hk595

The simplest way is to connect both the left and right channel speaker wires from your amp or receiver into the speaker wire connections on the back of the sub labeled "From Amplifier"

Then connect speaker wires from the adjacent terminals labeled "To Speakers"

What you are essentially doing now is using the internal crossover of the subwoofer where you determine at what frequency the sub begins to blend with the other speakers.

This is done with the Crossover knob just above the speaker wire connections.

Whatever you have this set to, the main volume will now raise and lower the volume of the sub and main speaker together but you can still make the sub louder or softer as compared to the main speakers by using the subs volume control.

Defeat all tone controls on your preamp or receiver and use only the crossover knob and sub volume until you achieve a seamless blend where you cannot tell where the sub is and it is an extension of the speakers from where you will be sitting.

TIP:
DO not evaluate from the sub itself because the room shape and acoustics are drastically different everywhere ESPECIALLY when you are dealing with bass frequencies.

Can I assume you know how to set this?

Appears that the map powering the sub woofer is fubar. Those amps take a beating usually from being over driven. Transistors are not like tubes that will just reach a limit, distort, and stay there. You push a transistor and it will self destruct since it knows no limits ala evel knevil.

Amps usually receive their signal through RCA wires. Your radio should have one pair of RCA outputs on the back. (red and white) You will adjust your subwoofer volume by adjusting the gains on the amplifer (little knob next to the RCA inputs of the amp), then you can turn up the bass on your radio.

I am having the exact same problem with my Harmon kardon PC speakers. It is related directly to the speakers (subwoofer). If you plug head phones into the headphone jack on the satellite speaker you will not hear the noise. I am still searching for a solution, I love these speakers!

does the sub have a power supply, ie a power cord, if it does this is do able if it does not, then it has no amp for the sub and would require that

you need the the satellite speakers hooked up in order for the sub to work. its not a standalone sub the volume control knob on the satellite speaker is what powers on the sub. i have extra speakers i wish i could give you cuz my sub just blew out.

does your new sub have a volume or gain control? If your getting the signal source from the line outs, then the volume level there is constant. You need to set it up so that it can be controlled by the main volume control.Or you might need a control pre-amplifier before your sub

there may be some problems with the wiring. either change the whole wiring or try to keep the speaker volume at low when they r idle. it will certainly help .

I'm not sure of anywhere online, but I actually have a spare set of the satellite speakers sitting at my house, from a unit I had previously (the sub went out). I can make sure they work and get them to you if you'd like, email me at drkstr (at) gmail (dot) com...
But I have only one spare...
I'm not sure about wherever else you can buy them online. I was actually lookin to buy another set when I stumbled on this thread, but they're kinda hard to find lol. The whole unit used to be $30 for a refurb'd model, but those places are out-of-stock currently.